Altruism (donating goods or services with no immediate benefit) and reciprocity (taking turns behaving altruistically) are important topics in the behavioral sciences because individuals forego immediate benefits to gain larger, long-term rewards. Whereas reciprocity is a common part of human sociality, data suggest that it rarely occurs in non-human social systems. The proposed study explores the hypothesis that the psychological complexity involved in tracking debts owed and favors given prevents reciprocity in non-human animals. Despite a large literature that examines the effects of numerical ability (quantifying reward amounts and delay to reward) and self-control on individual choice behavior, psychological constraints on altruism in non-human animals has largely been ignored. This project first tests individual cotton-top tamarins (Saguinus oedious) in three tasks--number discrimination, temporal discrimination, and self-control--to establish constraints in psychological ability. The tamarins then play cooperative games using reward systems falling within and outside of the previously determined constraints to examine their influence on cooperative behavior. This project provides an innovative synthesis of mechanistic and evolutionary perspectives to provide a novel framework upon which future experiments on altruism and cooperation can build.